Nuclear fuel assembly

ABSTRACT

A fuel assembly for a boiling water reactor comprising a plurality of fuel units ( 3 a,  3 b,  3 c,  3 d), stacked on top of each other, each one comprising a top tie plate ( 5 ), a bottom tie plate ( 6 ) and a plurality of fuel rods ( 4 a,  4 b,  4 c) arranged between the top tie plate and the bottom tie plate. The fuel units are surrounded by a fuel channel ( 9 ) with a substantially square cross section. At least some of the fuel units comprise fuel rods with different diameters and different fuel quantities. The fuel rods are adapted such that fuel quantity and lattice space are optimized laterally and axially in the fuel assembly.

TECHNICAL FIELD

[0001] The present invention relates to a nuclear fuel assembly with asubstantially square cross section for a boiling water reactor moderatedby light water, comprising a plurality of fuel rods extending between atop tie plate and a bottom tie plate and being surrounded by a fuelchannel.

BACKGROUND ART

[0002] In a nuclear reactor, moderated by means of light water, the fuelexists in the form of fuel rods, each of which contains a stack ofpellets of a nuclear fuel arranged in a cladding tube. A fuel bundlecomprises a plurality of fuel rods arranged in parallel with each otherin a certain definite, normally symmetrical pattern, a so-calledlattice. The fuel rods are retained at the top by a top tie plate and atthe bottom by a bottom tie plate. To keep the fuel rods at a distancefrom each other and prevent them from bending or vibrating when thereactor is in operation, a plurality of spacers are distributed alongthe fuel bundle in the longitudinal direction. A fuel assembly comprisesone or more fuel bundles, each one extending along the main part of thelength of the fuel assembly. Together with a plurality of other fuelassemblies, the fuel assembly is arranged vertically in a reactor core.The core is immersed into water which serves both as coolant and asneutron moderator.

[0003] Since the coolant in a boiling water reactor is boiling, a ratiobetween water and steam is formed which varies axially in the core. Atthe bottom of the core, the temperature of the coolant is lower than theboiling temperature and is thus in a single-phase state, that is onlywater. At the top of the core, where the coolant has reached the boilingtemperature, part of the water is transformed into steam, and thecoolant is thus in a two-phase state. The higher up in the core, thegreater is the percentage of steam in relation to the percentage ofwater. In the uppermost part of the core, the fuel rods are only coveredwith a thin film of water, outside of which steam mixed with waterdroplets is flowing, so-called annular flow.

[0004] If the thermal flow from a fuel rod becomes very great inrelation to the coolant flow, there may be a risk of dryout. Dryoutmeans that the liquid film becomes so thin that it is not capable ofholding together, but it breaks up and forms dry wall portions, whichlocally leads to a considerably deteriorated heat transfer between thefuel rod and the coolant water resulting in a greatly increased walltemperature of the fuel rod. The increased wall temperature may lead todamage with serious consequences arising on the fuel rods. The risk ofdryout exists substantially in the upper part of the fuel assembly.

[0005] Because of its lower density, steam is much inferior to water asmoderator, which during operation of the reactor means that the higherup in the fuel assembly, the worse the moderation. In the core the fuelassemblies are surrounded by water which gives a good moderation of fuelrods near the fuel channel. In fuel rods in the central parts of thefuel assembly, on the other hand, inferior moderation will occur. Aboveall the central parts of the upper part of the fuel assembly will havean insufficient moderation. The reactivity of the reactor depends on theratio of uranium to moderator. To obtain an optimum uranium-to-moderatorratio, the quantity of uranium should be smaller and the lattice space,that is, the free space between the fuel rods, should be larger in theupper part of the fuel assembly than in the lower part thereof.

[0006] Factors which are important to take into consideration whenoptimizing the fuel assembly are, in addition to reactivity and dryout,limitation of the linear load of the fuel rods, shutdown margin, andpressure drop.

[0007] A constantly recurring problem with boiling water reactors is howbest to optimize the fuel assembly both axially and laterally withrespect to uranium quantity and lattice space. Laterally, anoptimization may be made, for example, by the choice of the diameter ofthe fuel rods, the distances between the fuel rods, and the number offuel rods. A well-known method of achieving an axial optimization is toreplace some of the fuel rods by part-length fuel rods. Part-length fuelrods have a shorter axial extent than the traditional full-length fuelrods. Another method of achieving an optimization of the uraniumquantity both axially and laterally is to vary the enrichment of thefuel in the fuel rods, which is shown in the patent specification of DE40 14 861 A1. A fuel assembly is shown which has fuel rods withdifferent enrichment in different lattice positions and certain of thefuel rods have several different enrichment contents axially.

[0008] A disadvantage with the above-mentioned optimization methods isthat they are not capable separately to provide sufficiently efficientoptimization of fuel and lattice space. With a conventional fuelassembly, it is difficult to achieve a good optimization in a simplemanner. A solution to this problem is shown in PCT/SE95/01478 (Publ. No.WO 96/20483) which shows a flexible fuel assembly which can be optimizedin a simple manner, both axially and laterally. The flexible fuelassembly comprises a plurality of fuel units stacked on top of eachother, each comprising a plurality of fuel rods extending between a toptie plate and a bottom tie plate. The fuel units are surrounded by acommon fuel channel with a substantially square cross section.

[0009] The needs of axial and lateral optimization differ betweenvarious reactors and various operating conditions. It is, therefore,desirable to be able to offer, for each individual customer, a fuelassembly which is optimized for the special needs of each individualcustomer. One problem is that it may be very expensive to supplydifferent fuel assemblies to different customers since it requires alarge number of different components which are both to be manufacturedand be kept in stock.

SUMMARY OF THE INVENTION

[0010] The present invention relates to a fuel assembly which is simpleto optimize, both axially and laterally, based on the needs ofindividual customers, but which can still be manufactured in a simpleand rational, and hence economic, manner.

[0011] What characterizes a fuel assembly according to the inventionwill become clear from the appended claims.

[0012] A fuel assembly according to the invention may be composed offew, preferably three, standardized types of fuel rods, which aredistinguished from each other in that they have different externaldiameters and different uranium quantities. With three types of fuelrods and a large number of fuel units, preferably at least seven fuelunits in the fuel assembly, a very great freedom to optimize the fuelassembly, both axially and laterally, is obtained. The three differenttypes of fuel rods may, in turn, be combined into a number of differenttypes of fuel units with suitable properties for different levels in thefuel assembly and for different operating conditions. By distributingthe different fuel units at different levels, a desired axialoptimization may be achieved. Besides varying the distribution of thefuel, also the total quantity of fuel in the fuel assembly may be variedin a simple manner by varying the number of fuel rods of the differenttypes. In addition, certain positions in the lattice may be leftunoccupied, for example for improving the shutdown margin.

[0013] One advantage of the invention is that a large number ofdifferent fuel assemblies with different properties may be assembledfrom a small number of standardized units. At a small extra cost, a fuelassembly may be obtained which is tailored to the reactor and theoperating conditions under which it is to serve.

[0014] Still another advantage is that the number of fuel rods withdifferent enrichment levels may be limited, which means that the fuelrods become simpler and less expensive to manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 schematically shows one embodiment of a fuel assemblyaccording to the invention.

[0016]FIGS. 2a, 2 b, 2 c and 2 d show horizontal sections B-B, C-C, D-Dand E-E through the fuel assembly in FIG. 1

[0017]FIG. 3 shows a cross section of an absorber rod.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]FIG. 1 shows a fuel assembly according to the invention. Duringoperation, the fuel assembly is arranged vertically in the reactor core.The fuel assembly comprises an upper handle 1, a lower end portion 2 anda plurality of fuel units 3 a, 3 b, 3 c and 3 d stacked one above theother. The fuel unit comprise a plurality of fuel rods 4 a, 4 b and 4 carranged between a top tie plate 5 and a bottom tie plate 6. The fuelunits are stacked on top of each other in the longitudinal direction ofthe fuel assembly and they are stacked in such a way that the top tieplate 5 in one fuel unit is facing the bottom tie plate 6 in the nextfuel unit in the stack. A fuel rod contains fuel in the form of a stackof uranium pellets 8 arranged in a cladding tube 7. The fuel assembly isenclosed in a fuel channel 9 with a substantially square cross section.In this embodiment, the fuel assembly comprises four parallel stackswith ten fuel units in each stack.

[0019]FIG. 2a shows a section B-B through the fuel assembly in FIG. 1.The fuel channel 9 is provided with a hollow support member 10 ofcruciform cross section, which is secured to the four walls of the fuelchannel. In the central channel 11 formed of the support member 10,moderator water flows. The fuel channel with the support membersurrounds four vertical channel-formed parts 12 a, 12 b, 12 c, 12 d,so-called sub-channels, with an at least substantially square crosssection. The four sub-channels each comprises a stack of fuel unit. Eachfuel unit comprises 24 fuel rods arranged in a symmetrical 5×5 lattice.By a fuel rod position is meant a position in the lattice. All the fuelrod positions in the lattice need not be occupied by fuel rods.

[0020] The fuel assembly has three different types of fuel rods 4 a, 4 band 4 c. In FIGS. 2a-2 d the fuel rods 4 a are designated M and the fuelrods 4 a are designated P. The fuel rods 4 b are not marked in thefigures. A fuel rod 4 a has a diameter d₁. A fuel rod 4 b has a diameterd₂ which is about 8% larger than d₁ and contains about 15% more fuelthan the fuel rod 4 a. A fuel rod 4 c has a diameter d₃ which is about8% larger than d₂ and contains about 15% more fuel than the fuel rod 4b. By varying between the three fuel rod types in the different latticepositions, a great variation of fuel units may be created. The fuel rodswith the largest diameter, 4 c, have a relatively larger fission gasspace than the fuel rods with the smallest diameter, 4 a, in order thusto take into account different linear loads because of rod diameters andtypical neutron-flux ratios. It is not sufficient that the diameter islarger but also the height of the fission gas space should be larger.

[0021] In this embodiment, the fuel assembly is composed of fourdifferent types of fuel units 3 a, 3 b, 3 c, 3 d at ten differentlevels. FIG. 2d shows a section E-E through the fuel unit 3 a. The fuelunit 3 d is formed to fit into the lower part of the fuel assembly wherethe neutron flux tends to be high for a large part of the operatingcycle. This fuel unit almost exclusively comprises fuel rods of the 4 ctype, which is that of that fuel rods which has the largestcross-section area and contains most fuel. In the lowermost part of thefuel assembly, the significance of a reduced flow area because of thelarge cross-section area of the fuel rods is not so great since both themoderation and the cooling are good and the pressure drop is still lowbecause of a low steam content.

[0022] The higher up in the fuel assembly, the fewer are the fuel rodswith the largest diameter 4 c and instead the number of fuel rods with asmaller diameter 4 a and 4 b increases. FIG. 2b shows a section C-Cthrough the fuel unit 3 c and FIG. 2c shows a section D-D through thefuel unit 3 b. FIG. 2a shows a section B-B through the uppermost fuelunit 3 d, which comprises only fuel rods of types 4 a and 4 b, whichboth have a diameter and a fuel content which are smaller than those ofthe fuel rod 4 c. In addition, the lattice positions nearest the waterchannel 11 are unoccupied. One advantage of the unoccupied positions isthat the shutdown margin increases. In the upper part of the fuelassembly, the optimization of the fuel units takes place in order tominimize the risk of dryout and to obtain a low pressure drop.

[0023] To absorb part of the surplus reactivity in the fuel when it isfresh, certain of the fuel rods may contain a burnable absorber, forexample gadolinium oxide. Such a fuel rod will be referred to below asan absorber rod. The diameter of the absorber rod determines its burnuprate. The absorber rods 13 a, 13 b, 13 c are available in threedifferent sizes with three different diameters d₁, d₂, d₃ which are thesame as for the fuel rods. By arranging absorber rods with differentdiameters in the lattice, the content of burnable absorber may befinely-divided both axially and laterally with respect to reactivity,burnup behaviour and power distribution.

[0024]FIG. 3 shows an absorber rod 13 c in cross section. The absorberrod comprises a plurality of fuel pellets 15 and 8 a stacked on top ofeach other in a cladding tube 7 and a top plug 16 and a bottom plug 17seal the absorber rod. The fuel pellets 15 contain a certain part of aburnable absorber. The two end pellets 8 a in the absorber rod onlycontain fuel and lack burnable absorber. The end pellets in both thefuel rods and the absorber rods adjoin axial gaps which arise betweenthe fuel units in the stack. Because of the axial gap, the moderationand hence the reactivity become higher in the end pellets compared withthe other pellets in the stack. By not adding any burnable absorber tothe fuel in the end pellets, the end pellets in the fuel unit are burntup faster than other pellets. The burnup takes place at the beginning ofthe operating cycle while the total power of the fuel assembly is stilllimited by the burnable absorber. Since it is necessary in some way, forexample by a lower enrichment or by providing them with holes, to limitthe power in the end pellets of the fuel rods, it is an advantage thatall the end pellets are identical so that the manufacture is simplified.

[0025] In this embodiment, all the fuel units have the same kind oflattice. It is an advantage that all the fuel units have the samelattice because then the same bottom tie plates and top tie plates maybe used for the different fuel units, which minimizes the number ofcomponents which need to be manufactured and kept in stock. It is alsopossible, while maintaining the same lattice, to carry out optimizationsby limited displacements of the positions of the rods.

[0026] In another embodiment, fuel units at the same level in the fuelassembly may have different distribution of fuel rods. This may, forexample, be advantageous in a reactor where the fuel assembly issurrounded by water gaps with different widths. The moderation becomesdifferent depending on which gaps a fuel unit is facing, which may becompensated for by arranging fuel rods with larger or smaller diametersin lattice positions adjacent the gaps.

1. A fuel assembly for a boiling water reactor comprising a plurality of fuel rods (4), which fuel rods are arranged in a lattice and extend between a top tie plate (5) and a bottom tie plate (6) and are surrounded by a fuel channel (9) with a substantially square cross section, characterized in that the fuel assembly comprises a plurality of fuel units (3 a, 3 b, 3 c, 3 d) stacked one above the other, each one comprising a top tie plate (5), a bottom tie plate (6) and a plurality of fuel rods (4 a, 4 b, 4 c) arranged between the top tie plate and the bottom tie plate, at least certain of the fuel units (3 a, 3 b, 3 c, 3 d) comprise a plurality of fuel rods with different diameters and different quantities of fuel, the fuel rods are so arranged in the fuel units that the quantity of fuel and the lattice space are optimized both axially and laterally in the fuel assembly.
 2. A fuel assembly according to claim 1 , characterized in that the fuel assembly comprises at least three fuel rods (4 a, 4 b, 4 c) with different diameters (d₁, d₂, d₃) and different fuel quantities.
 3. A fuel assembly according to claim 1 or 2 , characterized in that the fuel assembly comprises a first fuel rod (4 a) with a first diameter (d₁), a second fuel rod (4 b) with a second diameter (d₂) which is at least 5% larger than the first diameter, and a third fuel rod (4 c) with a third diameter (d₃) which is at least 5% larger than the second diameter.
 4. A fuel assembly according to any of the preceding claims, characterized in that the fuel assembly comprises at least seven fuel units.
 5. A fuel assembly according to any of the preceding claims, characterized in that all the fuel units have substantially the same lattice.
 6. A fuel assembly according to any of the preceding claims, characterized in that the fuel assembly comprises a fuel unit (3 d) where at least one position in the lattice is unoccupied.
 7. A fuel assembly according to any of the preceding claims, characterized in that it comprises absorber rods (13 a, 13 b, 13 c) containing burnable absorber material, whereby at least two of the absorber rods have different diameters (d₁, d₂, d₃).
 8. A fuel assembly according to claim 8 , characterized in that the end pellets (8 a) of the absorber rods lack burnable absorber material. 